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Freeze casting

Nature Reviews Methods Primers volume 4, Article number: 28 (2024) Cite this article

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Abstract

When solutions and slurries are directionally solidified, complex dynamics of solvent crystal growth and solvent templating determine the final hierarchical architecture of the freeze-cast material. With continuous X-ray tomoscopy, it is now possible to study in situ intricate and otherwise elusive ice crystal growth and solvent-templating phenomena. Quantifying these phenomena both time-resolved and in three dimensions provides novel insights into the formation of performance-defining features of freeze-cast cellular solids at several length scales: the material’s pore morphology (first hierarchical level), the molecular, fibrillar and particle self-assembly of components in the cell walls (second level) and the cell wall surface structures (third level). The freeze casting process is attractive because the features of the final hierarchical material architecture — which determine the material’s structural, mechanical and physical properties — can be custom designed for a given application. Overall porosity, pore size, geometry, orientation, particle packing in cell walls and cell wall surface features can be tailored for applications in, for example, biomedicine, environmental engineering, catalysis, power conversion, and energy generation and storage.

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Fig. 1: Schematic of the freeze casting process and examples of structural features templated in freeze-cast polymers, ceramics and metals.
Fig. 2: An overview of freeze caster, mould designs, resulting domain shape and lamellar orientation in two and three dimensions.
Fig. 3: Dynamic structural characterization of crystal growth processes with tomoscopy.
Fig. 4: Overview of in situ and post-mortem techniques to characterize the dynamics of ice crystal growth and ice templating during solidification and the hierarchical material architecture that results ordered by length scale.
Fig. 5: Quantitative in situ characterization of the structure evolution, features formation and particle arrangements.
Fig. 6: Structure–property correlations.
Fig. 7: Example sample sectioning schemes for material analysis and mechanical testing.

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Acknowledgements

The authors gratefully acknowledge financial support through NASA awards 80NSSC18K0305 and 80NSSC21K0039 (U.G.K.W.) and NSF-CMMI award 1538094 (U.G.K.W.), the DFG Reinhart-Koselleck Project 408321454, Ba 1170/40 (F.G.-M) and BMBF award 05K18KTA (F.G.-M), the Alexander von Humboldt Foundation for a Humboldt Research Fellowship (K.Y.).

Author information

Author notes

  1. These authors contributed equally: Paul H. Kamm, Kaiyang Yin.

Authors and Affiliations

  1. Biomimetic Design Laboratory, Department of Physics, Northeastern University, Boston, MA, USA

    Ulrike G. K. Wegst

  2. Process Imaging Laboratory, Institute of Materials Science and Technology, Technische Universität Berlin, Berlin, Germany

    Ulrike G. K. Wegst, Paul H. Kamm & Francisco García-Moreno

  3. Process Imaging Laboratory, Institute of Applied Materials, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, Germany

    Paul H. Kamm & Francisco García-Moreno

  4. Laboratory of Micro and Materials Mechanics, Department of Microsystems Engineering, University of Freiburg, Freiburg, Germany

    Kaiyang Yin

Authors
  1. Ulrike G. K. Wegst
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  3. Kaiyang Yin
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  4. Francisco García-Moreno
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Contributions

Introduction (U.G.K.W., P.H.K., K.Y. and F.G.-M.); Experimentation (U.G.K.W., P.H.K., K.Y. and F.G.-M.); Results (U.G.K.W., P.H.K., K.Y. and F.G.-M.); Applications (U.G.K.W., P.H.K., K.Y. and F.G.-M.); Reproducibility and data deposition (U.G.K.W., P.H.K., K.Y. and F.G.-M.); Limitations and optimizations (U.G.K.W., P.H.K., K.Y. and F.G.-M.); Outlook (U.G.K.W., P.H.K., K.Y. and F.G.-M.); overview of the Primer (U.G.K.W., P.H.K., K.Y. and F.G.-M.).

Corresponding authors

Correspondence to Ulrike G. K. Wegst or Francisco García-Moreno.

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Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Reviews Methods Primers thanks Qunfeng Cheng, Hao Bai, Seog-Young Yoon, Lennart Bergström and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Related links

Ansys Granta Selector: https://www.ansys.com/en-gb/products/materials/granta-selector

Avizo: https://www.thermofisher.com/uk/en/home/electron-microscopy/products/software-em-3d-vis/avizo-software.html

Dragonfly: https://www.theobjects.com/dragonfly/index.html

Freezecasting.net: http://www.freezecasting.net/

Matricel: https://www.matricel.com/

Tomoscopy experiments database: http://resolution.tomoscopy.net

Tomoscopy.net: http://www.tomoscopy.net/

Supplementary information

Supplementary information

43586_2024_307_MOESM2_ESM.avi

Supplementary Video 1. Unidirectional solidification of a cylindrical freeze-cast specimen. Shown is the unidirectional solidification of a section of the cylindrical sample in a 2 mm diameter polyimide tube observed for 267 s. The video illustrates the dynamics of crystal growth and solution templating during freeze casting. The solidified ice crystals are shown in blue and the ice-templated sucrose phase in yellow. The solidification front advances with an average velocity of ~11 μm s−1. Domains are defined by differences in lamellar orientation. The process was imaged at one tomoscopy per second. The video plays at 10 frames per second. Video is reprinted from ref. 34, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

43586_2024_307_MOESM3_ESM.avi

Supplementary Video 2. Growth of an extracted single ice lamella. The video shows the crystal growth of a single ice lamella from one of the central domains in a unidirectionally solidifying 3% w/v sucrose in water solution. In the 267 s shown, different types of secondary instability form unilaterally on the side of the ice lamella, which faces the cold end of the mould; the sometimes transient nature of the instabilities is noteworthy. The lamella expands in thickness and width with increasing height owing to a decrease in the local cooling rate, \(\dot{C}\). The slight directional changes at about t = 70 s and t = 150 s from 4.4° to 10°, respectively, is paralleled by the formation of secondary instabilities and the lamella splitting into two. The video plays at 10 frames per second. Video is reprinted from ref. 34, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

43586_2024_307_MOESM4_ESM.avi

Supplementary Video 3. Evolution of ice-templated structure. The video shows the structural evolution of the ice-templated sucrose lamella for 160 s. It is noteworthy that the progressing freezing front pushes surface features, in this case a jellyfish cap array along with it parallel to the direction of solidification. Also shown is the formation of first attached, then detached, tentacle-like features. The video plays at nine frames per second. Video is reprinted from ref. 34, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

43586_2024_307_MOESM5_ESM.avi

Supplementary Video 4. Formation of a Jellyfish Cap array with Tentacles. Shown in this video at high magnification is the formation over 30 s of first attached, then detached, tentacle-like surface features extending from the jellyfish cap array parallel to the freezing direction. The video plays at ten frames per second. Video is reprinted from ref. 34, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

Glossary

Anisotropy

Anisotropic crystal growth is characterized by growth characteristics that depend on crystallographic direction. Freeze-cast materials tend to be anisotropic. Their solvent-templated structure and, as a result, also their properties vary with the direction of loading. When their properties are unique in three mutually perpendicular directions, the material is called orthotropic.

Applied cooling rate

The rate at which the temperature decreases per unit time, \({\dot{C}}_{\mathrm{app}}\), at the cold source.

Attenuation coefficient

A measure of how easily X-rays can penetrate a material, given by the fraction of incident photons in a mono-energetic beam that are attenuated per unit thickness of that material.

Dendritic ice crystal growth

Branched or tree-like crystal growth.

Directional solidification

Controlled crystal growth in a well-defined direction determined by the applied thermal gradient.

Domain

A volume of solid, akin to a grain, wherein ice crystals and templated lamellae exhibit the same orientation and alignment, respectively.

Extensional shear flow

The 9% volumetric expansion of water upon solidification into ice results in an interdendritic extensional shear flow. An additional thermal mould contraction can enhance this flow when batch processing. The extensional shear flow contributes to the preferential alignments of components during directional solidification.

Field of view

(FOV). The projected area imaged. The FOV corresponds to the region of interest, usually measured in pixels.

Freeze casting

Directional solidification of solvent-based solutions or slurries, followed by the removal of the solvent in its solid state by sublimation or solvent exchange.

Freezing front velocity

The speed of progression of the solid–liquid interface, in dendritic crystal growth defined by the crystal tip array.

Greyscale

A synonym for the range of voxel values within a slice, volume or tomographic data set.

Hierarchical pore architecture

Porosity at different length scales contributed by different levels of the hierarchy. In freeze-cast materials, the hierarchical pore architecture is frequently composed of particle porosity, porosity in the cell walls and porosity defined by the cell walls of the cellular solid.

Ice templating

The shaping of the solute phase by ice crystals growing from an aqueous solution or slurry.

Instability

For example, a crystal instability forms when a planar solid–liquid interface is perturbed and a stable array of dendrites starts to form.

Lamellar spacing

The periodic lamellar spacing defines how coarse or fine the structure of a freeze-cast material is. It is defined as λ = S + w, the sum of the short pore axis, S, and the cell wall thickness, w. The lamellar spacing depends on the local cooling rate, \(\dot{C}\).

Local cooling rate

The rate at which the temperature decreases per unit time, \(\dot{C}\), at a well-defined position within the sample.

Nucleation

Nucleation is the formation of a small cluster (or nucleus) of atoms. Once a critical number of atoms assemble, crystal growth begins. Homogeneous ice nucleation can occur in the liquid phase and heterogeneous nucleation on the cooled mould surface.

Partially faceted crystal growth

Crystal growth that is faceted in some directions and not faceted in others.

Phase contrast

Contrast in a radiograph or tomogram resulting from the difference in phase developed by beams as they pass through an object.

Phase separation

Two or more phases forming from a single phase mixture of components.

Pore size

The pore size is defined by its cross-sectional area, A, the length of the short, S, and long, L, pore axes, their aspect ratio, R, and the cell wall thickness, w.

Principal component analysis

(PCA). A statistical technique used to analyse the variability of a multidimensional data set. PCA is used to identify the dominant patterns in the data and analyse the 3D spatial orientation of the segmented objects. This is typically done by projecting the voxel coordinates of that object onto a lower-dimensional subspace that captures the most significant variations in the data.

Projections

Radiographs of an object acquired at a given angle of illumination that, when combined with many others, provide the data to numerically reconstruct the object. Typically, between 100 and 3,600 projections are used to reconstruct a tomogram.

Thermal gradient

G, the rate at which the temperature varies per unit distance in a well-defined direction, a 3D vector.

Tomogram

Originally, a 2D slice through an object reconstructed computationally from a sinogram. Now often used to refer to the 3D reconstructed sample volume.

Tomograms per second

Unit of temporal resolution, used for the recording speed of continuous full tomograms in a tomoscopic experiment.

Voxel

Abbreviation for volume element; the basic unit of a 3D digital representation of a volume or object. Voxel size should not be confused with spatial resolution.

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Wegst, U.G.K., Kamm, P.H., Yin, K. et al. Freeze casting. Nat Rev Methods Primers 4, 28 (2024). https://doi.org/10.1038/s43586-024-00307-5

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